Friday, July 31, 2009

Software Defined Radio—the Basic Architecture

A software defined radio basically runs a set of programs such as transmitter/receiver algorithms on a processor. The baseband signal processing is implemented on an FPGA or DSP. An antenna provides the RF signal which is then sampled by an analog to digital convertor.

In this article the basic concepts are outlined. The RF signal from the antenna goes through a bandpass filter and amplification. The resulting signal is mixed with a locally generated RF frequency to create an Inphase and a Quadrature phase (90 degrees shifted) signal. These signals go through a lowpass filter and then into an analog-to-digital convertor. The sampling rate is typically fixed so it must be set at a rate that can sample a sufficient number of samples to recreate the signal properly across the entire range.

For reception an FPGA or host computer processor takes the signals and applies signal processing to break the signal into its component bits and symbols or to find some other set of parameters. For transmission, the processor works to create the waveforms that go back through the chain to be transmitted.

The key to software defined radio adoption in today’s world is the standard PC is now sufficiently powerful enough to perform the waveform processing. Of course DSPs and FPGAs enhance the performance of the system, but still the cost of a PC-based solution is far cheaper than that of a dedicated, proprietary system which is commonly found in military applications such as the JTRS program.

The need for more spectrum raises the need for cognitive radio techniques. Cognitive radio techniques address this issue. To implement a cognitive radio system one needs the following components:

1. Location sensors—establish its position
2. Spectral monitoring—monitor the network for open channels
3. Control software—negotiate the use of the spectrum between users

Best regards,
Hall T.

Friday, July 24, 2009

SDR Architectures and Systems – Software Communications Architecture

Software Defined Radio technology and open standards promise to drive down costs and increase the use of wireless technology in our everyday lives in the same way that the PC and standardized operating systems did for the computer industry.

In the world of software defined radio, the Software Communications Architecture developed by the SDR Forum and the Object Management Group defines how the system loads the waveforms, runs applications, and networks with other systems. The SCA focuses on the military’s JTRS program which seeks to combine the wide range of military radios into a common set using SDR techniques.

The SCA provides an open architecture that can handle multiple radios. These are interoperable over a wide range of frequencies and allow for other technologies to be included. The SCA also fosters software reusability.

The purpose of SCA is to provide portability of applications between different systems, leverage commercial standards to reduce development time, reduce software development through reuse of software modules, and build a set of architectures for commercial implementations. The SCA standard comes with an API to help define the relationship between waveform applications and the software defined radio system.

With additional standards and frameworks comes additional overhead. This additional overhead must be offset by additional increases in the performance of the core silicon. In this article, the author discusses the rise of high-level design tools and the rise of FPGA’s as the key to offset the additional overhead of frameworks in the SCA architecture.

Finally, reference architectures for SCA are numerous. One example can be found here and here.

Best regards,
Hall T.

Friday, July 17, 2009

Cognitive Radio – the First Model of Spectrum Sharing—Command and Control

As we blogged about two weeks ago (the 4th of July holiday intervened), there are three models of Spectrum Sharing - command and control, exclusive use, and unlicensed use. The first model is Command and Control. This has been the predominant form of spectrum usage. The governmental authorities divide up the spectrum in specific frequencies and place requirements and standards on the usage of that spectrum. Frequency allocation is an exclusive property of a national sovereignty in the same way as water, land, and mineral rights.

The command and control approach dates back to the early days of RF usage in which exclusive rights were given to a certain usage in order to achieve acceptable quality of signal given inherent interference. While this may no longer be necessary, there are benefits to the command and control approach in that it provides a level of standardization that many technologies require to be successful. Also, some technologies that may not achieve profitability on their own can still be provided through exclusive frequency licensing, if deemed necessary for the society.

Numerous papers describe the challenges that the command and control system faces, the most common being the ever increasing demand for spectrum by new users and new uses. The military provides a case study of the use of a command and control frequency allocation system challenged by the demand for more use of finite spectrum. In addition, the technical, geographical, and operational factors increase the need for awareness of the spectrum and its usage.

Best regards,
Hall T.

Wednesday, July 01, 2009

Cognitive Radio – Three Models for Spectrum Sharing

Continuing in our series on Cognitive Radio, I found a nice summary primer on Cognitive Radio and Software Defined Radio here. The emergence of DSP and higher speed analog-to-digital convertors gives Software Defined Radio techniques an advantage over the traditional demodulator block.

The paper goes on to describe three models for spectrum sharing. They are command and control, exclusive use, and unlicensed use. As for exclusive use, from time to time the US government auctions off spectrum to private companies who can do what they want with the spectrum – similar to property rights. Command and control has been the standard for spectrum usage in the US as bandwidth is licensed to specific users for a specific use case. Finally, the ISM band is an example of unlicensed use in which spectrum is made available on a first-come, first-served basis. There is no guarantee of interference protection.

The paper goes on to discuss how Cognitive Radio techniques must be able to work in all three of these cases. We’ll explore these three methods in the coming weeks.

Best regards,
Hall T.